Magnetohydrodynamic Phenomena in the Sun and Solar Wind

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This grant will support theoretical research in 3 areas, - The solar magnetic field is continuously regenerated and reconfigured by a hydromagnetic dynamo in the solar interior. The field emerges at the surface in the form of thin flux ropes and active regions. The strength and topology of the interior field are not directly observable; neither is the depth at which it fragments into flux tubes. However, by computing the effect of the field on the acoustic oscillation frequencies it is possible to use helioseismology to constrain the properties of the interior field. The magnetic field emerges from the photosphere and plays a dominant role in the structure and energetics of the solar corona. Energy is stored in the field by photospheric motions; these motions may cause current sheets to develop. Dissipation of these current sheets can heat the corona in a patchy or episodic fashion which is masked by low resolution observations. Energy stored in the field can also be released by magnetohydrodynamic instabilities. The magnetic field also plays a role in the origin and propagation of shock waves in the corona and heliosphere. Shocks can accelerate particles; the resulting nonthermal particle distribution can be unstable to the growth of waves, which modify the upstream medium so that the shock evolves. These ideas may explain observed evolution of shocks in the solar wind. All of the above is very relevant to solar terrestrial relations; the origin of the magnetic field, the heating of the corona and the propagation of the shocks all have impact on the terrestrial environment.